I don't know that I'd agree that a 1% change per difficulty adjustment is going to be enough.
All of the baked in constants are of course subject to tuning before final implementation. I used 90% and 1% as examples.
Granted.
Upon further consideration, I think that this might be a great way to move the soft limit, but I still think we need to have a (much higher) hard limit. For the reasons that I stated before, and others, a hard limit removes many of the incentives for big players to engage in anti-competitive activities; since it puts a very real limit to the long term effectiveness of such underhanded methods.